Generally, a voltage controlled oscillating circuit in which a micro strip (.mu.-strip) resonator is adopted is low in its Q-factor of the output, and therefore, much influence is received from the load. Therefore, in the conventional voltage controlled oscillating circuit, there is provided a multi-stage buffer amplifying circuit.
FIG. 1 illustrates a conventional circuit of the VCO, and as shown in this drawing, the VCO includes a .mu.-strip resonance circuit 60, an oscillation amplifying circuit 10, a two-stage buffer amplifying circuit 20, and an oscillation output impedance matching circuit 30.
The .mu.-strip resonance circuit 60 includes: a varactor diode VD1 with its capacitance .varied in accordance with an external tuning voltage V.sub.T, a capacitor C23 connected to the varactor diode VD1 in series, and a micro strip line S1, thereby determining the resonance frequency. The resonance frequency signals which are selected by the .mu.-strip resonance circuit 60 produce oscillations through capacitors C1 and C4 and an oscillating transistor Q1 of the oscillation amplifying circuit 10. The oscillation output is supplied through a coupling capacitor C12 to the two-stage buffer amplifying circuit 20.
The oscillating transistor Q1 is connected at its base to bias resistors R2 and R4, and is also connected at its emitter bias resistor R6. A B+ circuit voltage which is supplied to the bias resistors R2, R4 and R6 and to the collector of the oscillating transistor Q1 is supplied through an AC blocking RF choke coil L1 and a capacitor C2 for bypassing the noise of the power source. A capacitor C3 provides a ground-bypassing path and forms a common collector for the oscillation amplifying circuit 10.
The two-stage buffer amplifying circuit 20 includes two transistors Q2 and Q3 which are connected to bypassing capacitors C5-C7 and to bias resistors R1, R3, R5 and R7. The B+ circuit voltage is supplied through an RF choke coil L3.
The output of the two-stage buffer amplifying circuit 20 is supplied through the output impedance matching circuit 30 to the load, and the output impedance matching circuit 30 includes a coil L5 and capacitors C8 and C9.
However, the conventional VCO circuit as described above includes a two-stage buffering circuit such that the output of the VCO does not receive the load pulling influence from the load, and therefore, the constitution of the circuit becomes complicated, with the result that the bulk of the circuit is magnified, and the manufacturing cost is increased.